1. Field of the Invention
The present invention relates to an image forming apparatus for forming images on a recording medium by the electrophotographic process.
2. Description of the Related Art
The electrophotographic process has already found wide use in copying machines, facsimile systems, page printers and the like for forming images.
The electrophotographic process comprises the steps of uniformly charging the surface of a photosensitive member, exposing the surface of the photosensitive member to light in accordance with image data to form a latent image, developing the latent image with a toner image by depositing toner on the latent image, transferring the toner image onto recording paper, and fixing the transferred image to the paper.
Image forming apparatuses for practicing the electrophotographic process are adapted that when the power source is turned on, the operation values of various components or portions for determining the conditions for the process, such as the surface potential of the photosensitive member, amount of exposure and toner concentration, are set to initial values which are determined to give copy images of standard density.
Further copying machines, for example, have an operation key for specifying the density of copy images to be formed. With such a copying machine, electrophotographic process conditions, such as the amount of exposure, are altered according to the density specified. by the operation key, whereby copy images are formed with a density higher or lower than the standard density.
However, when used for a long period of time, the copying machine undergoes changes, which include adhesion of impurities to the photosensitive member, abrasion of the member, deterioration of the developer, deterioration of the exposure lamp, soiling of the optical system and alteration in the circuit constant of the control circuit.
For this reason, the conventional copying machine when used for a long period of time fails to produce copy images of standard density when components are set to initial operation values. More specifically, the copying machine has the problem of becoming unable to give a proper image density when the machine is set in the standard mode wherein no density adjustment is made by the operator.
This problem may be overcome by detecting various state values associated with the electrophotographic process conditions by suitable sensors every time a predetermined number of copies has been made or every time a predetermined period of time has elapsed and adjusting the process conditions based on the detected values for image adjustment.
Nevertheless, the time when image adjustment is made by detecting the state values with the sensors is not always the time when the image adjustment is needed. Consequently, the machine will be held out of copying operation while unnecessary image adjustment is executed and therefore still has the problem of being inefficient to use.